In EUV lithography apparatuses, reflective optical elements for the extreme ultraviolet (EUV) wavelength range (at wavelengths of between approximately 5 nm and approximately 20 nm) such as, for instance, photomasks or mirrors based on reflective multilayer systems are used for producing semiconductor components. Since EUV lithography apparatuses generally have a plurality of reflective optical elements, the latter have to have a highest possible reflectivity in order to ensure a sufficiently high total reflectivity. The reflectivity and the lifetime of the reflective optical elements can be reduced by contamination of the optically used surfaces of the reflective optical elements, which arises on account of the short-wave irradiation together with residual gases in the operating atmosphere. Since a plurality of reflective optical elements are usually arranged one behind another in an EUV lithography apparatus, even relatively small contaminations on each individual reflective optical element affect the total reflectivity to a relatively large extent.
Contamination can occur on account of moisture residues, for example. In this case, water molecules are dissociated by the EUV radiation and the resulting free oxygen radicals oxidize the optically active surfaces of the reflective optical elements. A further source of contamination is polymers, which can originate for example from the vacuum pumps used in EUV lithography apparatuses, or from residues of photoresists which are used on the semiconductor substrates to be patterned, and which lead under the influence of the operating radiation to carbon contaminations on the reflective optical elements. While oxidative contaminations are generally irreversible, carbon contaminations, in particular, can be removed, inter alia, by treatment with reactive hydrogen, by virtue of the reactive hydrogen reacting with the carbon-containing residues to form volatile compounds. The reactive hydrogen can be hydrogen radicals or else ionized hydrogen atoms or molecules.
If the light source provided in the EUV lithography apparatus generates EUV radiation on the basis of a tin plasma, tin and, if appropriate, zinc or indium compounds (or generally metal (hydride) compounds) occur in the vicinity of the light source and can attach to the optically used surface of, for example, a collector mirror. Since these substances generally have a high absorption for EUV radiation, deposits of these substances on the optically used surfaces lead to a high loss of reflectivity, for which reason these substances should be removed with the aid of suitable cleaning methods, for example with the aid of reactive (atomic) hydrogen or with the aid of a hydrogen plasma. Hydrogen can also be used to protect the optical surfaces arranged in the region of the light source against sputtering or etching away by tin ions. In this case, a hydrogen plasma is typically likewise generated on account of the EUV radiation present and the interaction of the hydrogen with ions and electrons. Optical elements in the vicinity of the light source or the reflective multilayer systems thereof should therefore also be stable in relation to degradation if they are permanently exposed to hydrogen plasma.
In order to protect a reflective multilayer system against degradation, it is known to apply a protective layer system to the multilayer system. Degradation is understood to mean contamination effects such as e.g. the growth of a carbon layer, oxidation, metal depositions, etc., but also the delamination of individual layers, the etching away or sputtering of layers, etc. In particular, it has been observed that under the influence of reactive hydrogen which is used for cleaning or which can arise on account of the interaction of the EUV radiation with hydrogen present in the residue atmosphere, detachment of individual layers, in particular close to the surface of the multilayer system, can occur.
US 2011/0228237 A1 discloses providing, for the purpose of protecting the reflective multilayer system, a protective layer system comprising at least two layers, of which one layer comprises a material selected from the group SiO2, Y2O3 and ZrO2 and a further layer comprises a material selected from a group comprising silicon oxide (having different stoichiometric ratios), Y and ZrO.
DE 10 2011 076 011 A1 discloses an optical element for reflecting EUV radiation comprising a protective layer system having a topmost layer composed of silicon carbide or ruthenium, wherein the protective layer system has a thickness of between 5 nm and 25 nm. At least two alternatively arranged layers of, firstly, carbon or ruthenium and, secondly, a carbide or a boride or a material from the group silicon carbide, silicon nitride, boron carbide or carbon are provided below the topmost layer. With the aid of the protective layer system, the multilayer system arranged underneath is intended to be protected in particular against reactive hydrogen having a kinetic energy of a few hundred electronvolts, which could otherwise penetrate into the multilayer coating and lead there to detachment of the topmost layers, in particular of silicon layers.